A poly lactic acid resin is a biomass polymer and therefore has been drawing attention in recent years against the background of the depletion of petroleum resources, the reduction of carbon dioxide emissions, and the like.
However, poly lactic acid itself is readily burned and thus is difficult to be used for members that require flame retardancy, such as electrical and electronic applications. In addition, the poly lactic acid has a low crystallization rate and is unlikely to be crystallized by a common film forming procedure. Thus, a film composed of a resin composition containing the poly lactic acid has a problem of poor heat resistance. For example, such a film is thermally deformed at about 60° C. or more that is a glass transition temperature of the poly lactic acid and cannot keep a film shape.
The poly lactic acid to be used for casings of home electric appliances, molded parts of automobiles, and the like may be required to have flame retardancy. Such poly lactic acid is further required to be halogen-free in order to reduce the environmental load.
For providing desired flame retardancy and heat resistance to the poly lactic acid resin, the following methods and the like have been developed.
For example, there has been developed a method of providing the flame retardancy and heat resistance by the addition of a phosphorus-containing or nitrogen-containing flame retardant into a mixture of a poly lactic acid resin and a heat resistant polymer such as a polycarbonate resin (Patent Documents 1 and 2). There has been also developed a method of providing the flame retardancy and heat resistance by heat treating a resin composition that is obtained by the addition of a flame retardant to a mixture of a poly lactic acid resin and an amorphous resin or a low-crystalline resin, during or after injection molding at a particular temperature to highly crystallize the poly lactic acid resin (Patent Document 3).
As the flame retardant effective for the poly lactic acid, there have been developed flame retardants that do not accelerate hydrolysis of the poly lactic acid. For example, there have been developed a method of adding a flame retardant that contains phosphorus and nitrogen and that has a surface coated with a hydrophobic inorganic oxide (Patent Document 4) and a method of adding, as a flame retardant, an aromatic condensed phosphoric acid ester having good hydrolysis resistance (Patent Document 5).
However, each method cannot achieve sufficient effects on the flame retardancy and heat resistance when it is applied to films or sheets. In particular, there have not been developed many flame-retardant methods applicable to a poly lactic acid-containing film or sheet having a thickness of less than 200 μm, and there have been developed few methods of providing a film or sheet further having flexibility.
Commonly, a film or sheet having a smaller thickness is more difficult to satisfy a standard for flame retardancy (for example, UL-94 VTM standards). To address this, a flame retardant is required to be mixed in a larger amount. However, the flame retardant is a foreign matter to the poly lactic acid resin and thus has a problem of reducing mechanical characteristics of the poly lactic acid resin.
For example, a flame retardant such as ammonium polyphosphate and melamine polyphosphate has a high flame retardant effect on the poly lactic acid, but even small particles of the flame retardant have a particle diameter of about 5 μm and thus largely affect the film mechanical properties. Meanwhile, some metal hydrates as a flame retardant have a particle diameter of 1 μm or less, but such a metal hydrate is required to be added in a large amount in order to achieve the flame retardancy. Moreover, even a surface treated metal hydrate cannot avoid water adsorption, and thus has a problem of causing hydrolysis of the poly lactic acid.
There is another problem. That is, when a resin composition containing the poly lactic acid is melted to form a film or sheet using metal rolls, the resin composition adheres to the metal rolls to interfere with the formation of the film or sheet because the resin composition has a poor releasability from the rolls.